1. Field of the Invention
This invention generally relates to offshore structures and more particularly to mobile submersible drilling platforms for use in arctic waters of moderate to deep depths, such as are encountered in the Beaufort Sea continental shelf area north of Canada and Alaska.
2. Description of the Prior Art
Over the continental shelf area of the Beaufort Sea, movements of large ice covers occur most of the year. As the winter progresses, the thickness of the ice cover increases. The ice generally breaks up annually leading to the open water season which lasts about 60 days. Relocation of a submersible drilling structure from one offshore drilling site to another is practical therefore only during the open water season.
Mobile arctic drilling structures have already been proposed and patented. These are intended to be towed to the desired drilling site during the short open water season and then they are submerged to rest on the sea bottom in relatively shallow arctic waters.
The known structures have sloping ice-engaging outer hull skins capable of breaking up encroaching and moving ice covers. They are primarily designed to resist the ice load expected at a specific site which is characterized by a specific water depth and by particular bottom soil conditions.
In their configurations, the known structures might be able to drill a few exploratory wells at a first site through the limited sea bed access area available to them. Since such structures become ice-locked shortly after deployment, it becomes necessary to wait for the next open water season in order to relocate to a second prospective drilling site, which must have substantially the same depth and environmental conditions as those of the first site.
Ordinarily, if a few wells drilled on the first site show a promising oil find, there is a need to develop many other wells on the first site in order to determine the economic potential of the found underground oil reservoir. Because of their inherent structural limitations, the known arctic marine drilling structures must be relocated several times over the first site in order to size up the extent of the oil and gas bearing formation.
The need to relocate the known arctic drilling structures makes the overall cost of evaluating potential offshore arctic hydrocarbon formations prohibitively expensive and wasteful of precious time, as when it is necessary to remain idle during the winter season while waiting to relocate.
Therefore, it is an object of the present invention to provide a single mobile offshore drilling structure which by using its onboard equipment can drill about four dozen developmental and production wells at a single site, which is capable of being relocated to another prospective site having largely different environmental conditions within a water depth range of 20-60 meters, which can carry sufficient supplies for drilling up to eight wells and/or operate for one full year without the need for resupply, which experiences operating loads from encroaching ice covers that are substantially of the same order of magnitude over the structure's entire operating water depth range of 20-60 meters, and which has a multi-slope deflector section with two transitional walls that automatically protects the deck from damage by broken-up large ice slabs that are forced to ride up on and then to fall off from the hull.